A 100-pound propane tank atop a Manhattan high-rise ignited in a thunderous explosion yesterday, touching off a spectacular blaze that brought the Garment District to a halt.

Six people suffered minor smoke inhalation.

The fire at 500 Seventh Ave., an 18-story commercial building at 37th Street, spread choking smoke to an adjacent building at 512.

The two-alarm fire began about 5:30 p.m. at a rooftop construction site at 500, where five propane tanks were stored, said Assistant Fire Chief Frank Cruthers.

The construction crew had left about 4 p.m.

The two-alarm blaze was quickly extinguished, but thick smoke felled six workers next door at 512.

Fragments from the construction site were imbedded in the brick wall of 512 by the force of the explosion, Cruthers said.

It was unclear what set off the explosion, and an investigation was underway to determine if the site had all the proper permits.

“It certainly had the potential for a much greater impact at and above the level of the fire,” Cruthers said, referring to other propane tanks on the roof.

“We were quite fortunate.”

The force of the blast and the fury of the smoke and flames had Garment District workers aghast.

“After the boom, in just a matter of seconds, the sky was red – a solid red glow … then pitch black,” said office manager Pamela Robinson of Vir2l Design Studio across the street from the blaze. “I was terrified – I didn’t know if the building was shaking or if it was just me.”

UPS worker Orlando Munoz said the explosion sounded “like a bomb,” panicking people on the street below.

“They ran one way, and then ran back,” he said. “People were running all over.”

Hat designer Patricia Underwood said she and a co-worker saw the explosion and “huge flames.”

“We felt the explosion – it was quite a pop,” she said. “My friend said he felt the windows rattle on 40th Street,” some three blocks away.